EM78P458AP datasheet(22/76 Pages) EMC

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EM78P458/459

OTP ROM

4.3 TCC/WDT & Prescaler

An 8-bit counter is available as prescaler for the TCC or WDT. The prescaler is available for either the

TCC or WDT only at any given time, and the PAB bit of CONT register is used to determine the

prescaler assignment. The PSR0~PSR2 bits determine the prescale ratio. The prescaler is cleared

each time the instruction is written to TCC under TCC mode. The WDT and prescaler, when assigned to

WDT mode, are cleared by the WDTC or SLEP instructions. Fig. 5 depicts the circuit diagram of

TCC/WDT.

• R1(TCC) is an 8-bit timer/counter. The TCC clock source can be internal or external clock input (edge

selectable from TCC pin). If TCC signal source is from internal clock, TCC will increase by 1 at every

instruction cycle (without prescaler). Referring to Fig. 5, selection of CLK=Fosc/2 or CLK=Fosc/4

depends on the CODE Option bit CLKS. CLK=Fosc/2 if CLKS bit is "0", and CLK=Fosc/4 if CLKS bit is

"1".

• If TCC signal source is from external clock input, TCC will increase by 1 at every falling edge or rising

edge of TCC pin.

• The watchdog timer is a free running on-chip RC oscillator. The WDT will keep on running even after

the oscillator driver has been turned off (i.e. in sleep mode). During normal operation or sleep mode, a

WDT time-out (if enabled) will cause the device to reset. The WDT can be enabled or disabled at any

time during the normal mode by software programming. Refer to WDTE bit of IOCE0 register. Without

presacler, the WDT time-out period is approximately 18 ms

1 NOTE: VDD=5V,Setup time period = 18ms ± 30%.

VDD=3V,Setup time period = 22ms ± 30%.

This specification is subject to change without prior notice.

06.25.2004 (V1.4)

EM78P458AP Datasheet(PDF) 22 Page - ELAN Microelectronics Corp